The present invention relates to an electronically-controlled carburetor used for an internal combustion engine, or more in particular to an electronically-controlled carburetor in which the air-fuel ratio is corrected by electronic control for attaining a proper air-fuel ratio over all the ranges of engine operation.
In recent years, automotive electronics have made rapid progress, and more electronic devices are being introduced into fuel supply control systems.
Especially, an electronic fuel injection system of multi-injection type with injection valves individually provided in the neighbourhood of the intake port of the respective combustion chambers is in the limelight and finds wide applications. As well known, an electronic fuel injection system is such that the amount of air introduced into the engine is mechanically or electrically detected, and the signal representing the amount of air thus detected is used to control the electrical signal for controlling the opening of the fuel injection valve, thus controlling the air-fuel ratio. In the case where a control circuit for producing a fuel-amount-control-signal in accordance with the air amount signal is arranged such that the circuit constant thereof is variably selectable so that desired air-fuel ratio can be easily achieved by properly selecting the value of the circuit constant. Also, under a particular running condition such as engine cold start, or high-load operation, etc., a signal produced from means for detecting such a particular running condition is transmitted to the above-mentioned control circuit, thus making it possible to attain the desired air-fuel ratio for the particular running condition. In the future when the progress in the electronics may come to require a fine and exact regulation of the fuel supply system in combination of a microcomputer, such a microcomputer can be coupled with the above-mentioned electronic fuel injection system with comparative ease.
The control of air-fuel ratio in the carburetor, on the other hand, depends in many respects on mechanical or hydrodynamic techniques and fewer attempts have been made to control the air-fuel ratio by electrical techniques. The electrical control of the air-fuel ratio has been applied only in certain carburetors which employ what is called the closed-loop control of air-fuel ratio as disclosed in U.S. Pat. No. 4,135,482 for example; in which the actual air-fuel ratio of the mixture gas supplied to the engine is detected by detecting one component of the exhaust gas, and the deviations of the actual air-fuel ratio from a commanded air-fuel ratio are corrected by driving an actuator provided in the fuel path in the carburetor in response to an electrical signal supplied from the control circuit. In the closed-loop control system now being commercialized, an oxygen sensor made of a solid electrolyte of the zirconia group is used as means for detecting one component of the exhaust gas. It is well known that this oxygen sensor produces an output voltage which changes stepwise at or in the vicinity of the stoichiometric air-fuel ratio. This closed-loop system uses a ternary catalyzer capable of purifying the exhaust gas by oxidizing/reducing the obnoxious components of the exhaust gas such as CO, HC and NOx. In view of the fact that the purifying efficiency of the ternary catalyzer is very high only at or near the stoichiometric air-fuel ratio, the output characteristics of the oxygen sensor are utilized to control the actual air-fuel ratio very skillfully at or about the stoichiometric air-fuel ratio. Thus, the carburetor of closed-loop type could not help being arranged such that although the air-fuel ratio of the carburetor can be maintained at or about the stoichiometric value by electrical means, particular air-fuel ratios in other specific operating ranges have to be controlled by mechanical or hydrodynamic means of the carburetor per se irrespectively of the above-mentioned electrical means. Even if an attempt is made in the future to improve the exhaust characteristics and fuel economy or fuel consumption by coupling the carburetor of closed-loop type to a microcomputer with the advance of the electronics, it will be impossible to control the air-fuel ratio over the entire operating ranges by electrical means by simply coupling the carburetor to the micrscomputer. Thus, while the air-fuel ratio control in the electronic fuel injection system can be achieved over the entire operating range by simply coupling a microcomputer thereto as mentioned above, such a control is not easily available in the case of the carburetor. In order that the carburetor of closed-loop type may electrically control the air-fuel ratio in the operating ranges not covered by electrical techniques, actuators are required to be provided at various portions in the carburetor paths. This proportionally complicates the construction of the carburetor and greatly increases the cost thereof.